Measuring the concentration of substances in biological fluids is an important tool for the diagnosis and treatment of many medical conditions. For example, the measurement of glucose in body fluids, such as blood, is crucial to the effective treatment of diabetes.
Diabetic therapy typically involves two types of insulin treatment: basal, and meal-time. Basal insulin refers to continuous, e.g. time-released insulin, often taken before bed. Meal-time insulin treatment provides additional doses of faster acting insulin to regulate fluctuations in blood glucose caused by a variety of factors, including the metabolization of sugars and carbohydrates. Proper regulation of blood glucose fluctuations requires accurate measurement of the concentration of glucose in the blood. Failure to do so can produce extreme complications, including blindness and loss of circulation in the extremities, which can ultimately deprive the diabetic of use of his or her fingers, hands, feet, etc.
Multiple methods are known for determining the concentration of analytes in a blood sample, such as, for example, glucose. Such methods typically fall into one of two categories: optical methods and electrochemical methods. Optical methods generally involve spectroscopy to observe the spectrum shift in the fluid caused by concentration of the analyte, typically in conjunction with a reagent that produces a known color when combined with the analyte. Electrochemical methods generally rely upon the correlation between a current (Amperometry), a potential (Potentiometry) or accumulated charge (Coulometry) and the concentration of the analyte, typically in conjunction with a reagent that produces charge-carriers when combined with the analyte. See, for example, U.S. Pat. No. 4,233,029 to Columbus, U.S. Pat. No. 4,225,410 to Pace, U.S. Pat. No. 4,323,536 to Columbus, U.S. Pat. No. 4,008,448 to Muggli, U.S. Pat. No. 4,654,197 to Lilja et al., U.S. Pat. No. 5,108,564 to Szuminsky et al., U.S. Pat. No. 5,120,420 to Nankai et al., U.S. Pat. No. 5,128,015 to Szuminsky et al., U.S. Pat. No. 5,243,516 to White, U.S. Pat. No. 5,437,999 to Diebold et al., U.S. Pat. No. 5,288,636 to Pollmann et al., U.S. Pat. No. 5,628,890 to Carter et al., U.S. Pat. No. 5,682,884 to Hill et al., U.S. Pat. No. 5,727,548 to Hill et al., U.S. Pat. No. 5,997,817 to Crismore et al., U.S. Pat. No. 6,004,441 to Fujiwara et al., U.S. Pat. No. 4,919,770 to Priedel, et al., and U.S. Pat. No. 6,054,039 to Shieh, which are hereby incorporated in their entireties. The biosensor for conducting the tests is typically a disposable test strip having a reagent thereon that chemically reacts with the analyte of interest in the biological fluid. The test strip is mated to a nondisposable test meter such that the test meter can measure the reaction between the analyte and the reagent in order to determine and display the concentration of the analyte to the user.
FIG. 1 schematically illustrates a typical prior art disposable biosensor test strip, indicated generally at 10 (see, for example, U.S. Pat. Nos. 4,999,582 and 5,438,271, assigned to the same assignee as the present application, and incorporated herein by reference). The test strip 10 is formed on a nonconductive substrate 12, onto which are formed conductive areas 14,16. A chemical reagent 18 is applied over the conductive areas 14,16 at one end of the test strip 10. The reagent 18 will react with the analyte of interest in the biological sample in a way that can be detected when a voltage potential is applied between the measurement electrodes 14a and 16a. 
The test strip 10 therefore has a reaction zone 20 containing the measurement electrodes 14a,16a that comes into direct contact with a sample that contains an analyte for which the concentration in the sample is to be determined. In an amperometric or coulometric electrochemical measurement system, the measurement electrodes 14a,16a in the reaction zone 20 are coupled to electronic circuitry (typically in a test meter (not shown) into which the test strip 10 is inserted, as is well known in the art) that supplies an electrical potential to the measurement electrodes and measures the response of the electrochemical sensor to this potential (e.g. current, impedance, charge, etc.). This response is proportional to the analyte concentration.
The test meter contacts the test strip 10 at contact pads 14b,16b in a contact zone 22 of the test strip 10. Contact zone 22 is located somewhat remotely from measurement zone 20, usually (but not always) at an opposite end of the test strip 10. Conductive traces 14c,16c couple the contact pads 14b,16b in the contact zone 22 to the respective measurement electrodes 14a,16a in the reaction zone 20.
Especially for biosensors 10 in which the electrodes, traces and contact pads are comprised of electrically conductive thin films (for instance, noble metals, carbon ink, and silver paste, as non-limiting examples), the resistivity of the conductive traces 14c,16c that connect the contact zone 22 to the reaction zone 20 can amount to several hundred Ohms or more. This parasitic resistance causes a potential drop along the length of the traces 14c,16c, such that the potential presented to the measurement electrodes 14a,16a in the reaction zone 20 is considerably less than the potential applied by the test meter to the contact pads 14b,16b of the test strip 10 in the contact zone 22. Because the impedance of the reaction taking place within the reaction zone 20 can be within an order of magnitude of the parasitic resistance of the traces 14c,16c, the signal being measured can have a significant offset due to the I-R (current×resistance) drop induced by the traces. If this offset varies from test strip to test strip, then noise is added to the measurement result. Furthermore, physical damage to the test strip 10, such as abrasion, cracks, scratches, chemical degradation, etc. can occur during manufacturing, shipping, storage and/or user mishandling. These defects can damage the conductive areas 14,16 to the point that they present an extremely high resistance or even an open circuit. Such increases in the trace resistance can prevent the test meter from performing an accurate test.
Thus, a system and method are needed that will allow for confirmation of the integrity of test strip traces, for measurement of the parasitic resistance of test strip traces, and for controlling the potential level actually applied to the test strip measurement electrodes in the reaction zone. The present invention is directed toward meeting these needs.